Most apparatus for determining liquid levels in tanks use floatation devices. These floatation devices accurately gauge volume but do not accurately gauge the weight of the fluid. One reason is that volume varies with temperature. Another reason is that it is only possible to equate a volume of liquid to its weight if the liquid is homogeneous.
For example, oil is treated at high temperatures and then pumped into storage tanks. Gauges that rely upon floatation devices will give one reading for the oil when it first flows into the tank after undergoing the heat treatment and a different reading after the oil has sat in the tank and had the opportunity to cool. Knowing the number of gallons of oil is not sufficient to provide an indication of weight unless there is additional information provided regarding the grade of oil. Light oil weighs approximately 6.8 pounds per gallon. Heavier oil may weigh in excess of 8.66 pounds per gallon. This has particular application for continuous flow treatment processes. With such processes, stopping to allow the oil to cool is disruptive to the process. As soon as the tank is filled to a predetermined level, a valve controlling flow into the tank is closed and a valve controlling flow out of the tank is opened to allow the contents of the tank to be pumped out.
Another example relates to the transportation of crude oil. In order to prevent overloaded transport vehicles from damaging public highways, every jurisdiction has highway load limits. Crude oil generally consists of a mixture of water and oil, the proportions of which frequently vary. In the absence of information regarding the composition of the crude oil, it is difficult, if not impossible, for the operator to accurately determine the amount of crude oil that can be transported while still staying within legal load limits.